The invention relates to a device for continuously filtering liquids, comprising an upright container portion that, in cross-section, is rectangular and has an inner wall rectangular in cross-section, whereby the filter chamber receives granular bulk material as a filter bed, and further relates to a method for continuously filtering liquids, especially for treating waste water with high solid and suspended material component such as industrial waste water, for treatment of water for industrial use taken from ground water and surface water, recirculating water, cooling water, and also swimming pool water as well as treatment of potable water.
In the prior art continuous sand filters are known which, for example, provide a radial filtration of the raw water through the filter bed. The raw water is then guided into a centrally arranged distribution chamber, flows via lamellas into the filter layer, whereby the filter layer is subjected to flow from the interior to the exterior. The filtrate leaves the filter layer via special filter nozzles arranged in the outer container wall and is then collected in an exterior annular chamber as the filtrate.
EP-A 0 291 538 discloses a sieve fabric arranged at the inner wall of the cylindrical annular chamber having a mesh width of 10 to 200 micrometer as a micro filter. The heavy, flocculated dirt particles collected at the micro filter are guided downwardly and are introduced into a cone-shaped removal chamber into the flowing filter bed. The micro filter is thus continuously hydraulically cleaned from the interior hollow space by use of a mammoth pump which is operated by pulsating introduction of compressed air. However, it was shown that the device expenditure of the conventional sand filter due to the hydraulic cleaning system is very high, which system is comprised of equidistantly spaced tubes arranged axis-parallel to one another. At their upper end they are fastened to a rotatably support star. Each of the tubes furthermore comprises a number of outlet jets that are directed at a slight slant onto the mantle surface of the sieve fabric.
Furthermore, the conventional sand filter may be able to handle for short periods of time flocculation of the raw water, but for extended periods of flocculation the outlet jets of the hydraulic cleaning system will become clogged and, also, clogging of the openings of the sieve fabric in the interior hollow space of the annular cylindrical container portion can be observed. At the same time, it is also possible that flocculation of the raw water in the filter bed of the conventional sand filter will occur which also will result in clogging so that in the filter layer a pressure loss is to be observed and the risk of penetration during filtration is present resulting in dirt particles reaching the filtrate side. It is often observed that the flocculation of the raw water results in a clogging action especially in the area of the filter bed which is close to the hydraulic ring system.
The solid particles retained within the pores of the filter bed reduce, of course, the permeability of the filter layer so that for the same load amount the pressure loss in the filter layer of conventional sand filters will increase. However, these processes are limited because the pressure loss cannot be increased indefinitely and the permeability cannot be reduced indefinitely. When the pores will clog because of deposits, the free pore diameter also becomes so small that the deposited dirt particles are torn off by shearing forces and will be entrained by the outflowing filtrate to the filtrate side, a situation that must be prevented at all cost.
In order to avoid a clogging of the conventional sand filters, for example, according to EP-A 0 291 538, sedimentation stages are arranged upstream of the conventional sand filter in order to (have) a precleaning action and thus prevent the risk of flocculation of the raw water in the conventional sand filter. They require a further control and additional coordination of the filtration steps, an expenditure that is to be avoided.
It is an object of the present invention to eliminate the disadvantages of the prior art. Furthermore, it is desirable to provide a device for continuous filtration of liquids which is especially suitable for cleaning industrial waste water or raw sewage with high solid load of any kind. Solid load in the context of the invention refers to, in addition to solid materials, also large volume flocculates or flocculate-like particles which conventionally have the tendency to quickly clog the filter layer areas facing the inlet of waste water. Furthermore, it is desirable when the device expenditure of the apparatus for continuous filtration of liquids is maintained minimal in order to ensure a high operational safety without the use of complicated control devices of the filtration steps. Furthermore, it would be very advantageous to provide a device with sufficiently constant permanent filtration capability that is independent of the type and composition of the waste water to be filtered.